MRI method for determining a magnetic field map from a bo reference scan and a wassr scan

Abstract

The invention provides for a medical imaging system (100, 300). The medical imaging system (100, 300) comprises a processor (104). Execution of machine executable instructions(120) causes the processor (104) to: receive magnetic resonance data, wherein the magnetic resonance datacomprises B0 field data (122) of a reference scan for a plurality of voxels and water saturation data (124) of a WASSR scan for a subset of voxels of the plurality of voxels, the water saturation data (124) comprising data of a limited number of sample points; determine a local absolute water saturation frequency (130) for each voxel of the subset using the water saturation data (124) of the WASSR scan; and re-construct a field map (132) comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction comprises determining relative frequency differences between the voxels using the B0 field data (122) of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies (130) of the subset.

Description

TECHNICAL FIELD[0001]The invention relates to magnetic resonance imaging, in particular it relates to a reconstruction of a field map.BACKGROUND OF THE INVENTION[0002]A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field or the main magnetic field. Various quantities or properties of the subject can be measured spatially using MRI. For example, the density of hydrogen protons can be measured. Various NMR spectrographic techniques can also be used spatially. However, often times a compound or metabolite is so dilute that as a practical matter it is not possible to make NMR spectrographic measurements directly.[0003]In theory, B0 should be a constant and homogeneous filed. However, considering an actual MRI system acquiring magnetic resonance data from a subject within an imaging zon...

AI Technical Summary

Benefits of technology

This invention describes a way to reconstruct a field map accurately and quickly using magnetic resonance imaging (MRI). The process involves identifying local water saturation frequencies for each voxel, which are directly related to the local frequency of the B0 field. This enables the system to better control the water saturation and improve the quality of the MRI image. The technical effects include improved accuracy and speed in field map reconstruction.

Problems solved by technology

However, often times a compound or metabolite is so dilute that as a practical matter it is not possible to make NMR spectrographic measurements directly.

The large value of gyromagnetic coefficient may cause a significant frequency shift even for a few parts per million field inhomogeneity, which in turn may cause distortions in both geometry and intensity of the MR images.

However, even with an ideal magnet, small inhomogeneities may always remain, e.g. due the susceptibility of the imaging subject.

Deviations in the order of a Hz may directly spoil the outcome of CEST imaging making the results unreliable.

However, a major drawback of WASSR is the long time required for data acquisition, since the WASSR data acquisition takes in the order of minutes.

Images